Why didn’t they search the property with ground penetrating radar 15 years ago? It’s not exactly new technology.
One reason that comes to mind for not using the ground penetrating radar 15 years ago is simply cost. The $$$ per hour for operating the radar in the field is just the start. In addition to that, the engineering $$$ can rack up impressively. I can see costs pushing to $10K pretty easily. A second possibility is that the investigators at the time simply did not know of the technology.